The existence of multiple insulin receptor substrate (IRS) proteins, IRS-1 IRS-2, IRS-3 and IRS-4, has raised the question of what extent these proteins play redundant versus complementary or unique roles in insulin and IGF-1 signaling. The studies proposed here are aimed at answering this question. Previous studies of 3T3 cells derived form IRS-1 (-/-) mice demonstrate that while IRS-1 and IRS-2 are interchangeable with regards to certain aspects of IGF-1 signaling, ie. activation of phosphatidylinositol 3-kijnase (PI 3-K) , activation of mitogen-activated protein (MAP) kinase, and induction of c-fos and egr-1, there are certain signaling events that are mediated by IRS-1 but not IRS-2 such as IGF-1 induced mitogenesis. These IRS -1 (-/-) cells provide a valuable tool to determine the degree of redundancey between IRS-1 and other IRS proteins, IRS-3 AND IRS-4. Cells derived from mice lacking other IRS proteins IR-2,IRS-3 AND IRS-4, will also be used to compare their roles in IGF-1 signaling once the animals become available. The IRS-deficient cells will also be transfected with PPARgamma to create adipocyte-like cells in order to examine the role of IRS proteins in the metabolic actions of insulin and IGF-1. Finally, these studies will also utilize chimeric IRS proteins in an attempt to identify regions of the IRS proteins responsible for mediating specific aspects of insulin and IGF-1 signaling as well as the proper subcellular localization of the various IRS family members and interactions with other SH2 domain containing and non-SH2 signaling molecules.